The present invention relates to an ultrasound diagnostic apparatus, and in particular, to an ultrasound diagnostic apparatus which performs transmission and reception of ultrasonic waves using a plurality of inorganic piezoelectric elements and a plurality of organic piezoelectric elements.
Conventionally, in the medical field, an ultrasound diagnostic apparatus using ultrasound images has been put to practical use. Generally, in this type of ultrasound diagnostic apparatus, an ultrasonic beam is transmitted toward the inside of a subject from an ultrasound probe, an ultrasonic echo from the subject is received by the ultrasound probe, and a signal of such reception is electrically processed, thereby producing an ultrasound image.
In recent years, in order to perform more accurate diagnosis, there has been an attempt to use harmonic imaging in which harmonic components generated by the distortion of an ultrasonic waveform due to nonlinearity of a subject are received and imaged. For example, in an ultrasound diagnostic apparatus disclosed in JP 2010-201120 A, it has been suggested that a reception signal obtained from an ultrasound probe and a delay signal obtained by delaying the phase of the reception signal using an analog delay circuit are added to remove a fundamental component from the reception signal and extract a harmonic component. An ultrasound image which accurately reflects the inside of the subject can be produced by the extracted harmonic components.
As an ultrasound probe suitable for harmonic imaging, for example, as disclosed in WO 2008/010509 A, an ultrasound probe which is formed by laminating an inorganic piezoelectric element and an organic piezoelectric element has been suggested. An organic piezoelectric substance which is used for the organic piezoelectric element responds to harmonic components of ultrasonic waves with high sensitivity, and by transmitting high-power ultrasonic beams from the inorganic piezoelectric element of the ultrasound probe, the harmonic components can be received by the organic piezoelectric element. Furthermore, fundamental components having an excellent S/N ratio can also be received by the inorganic piezoelectric element. In this way, reception signals having different properties from each other are obtained from the inorganic piezoelectric element and the organic piezoelectric element, and an ultrasound image suitable for diagnosis can be produced by adding a high-definition ultrasound image produced based on the reception signals from the organic piezoelectric element to an ultrasound image having an excellent S/N ratio and produced based on the reception signal from the inorganic piezoelectric element.
However, the organic piezoelectric substance of the organic piezoelectric element has electric capacitance smaller than that of the inorganic piezoelectric substance of the inorganic piezoelectric element. In general, intensity N of noise caused by a piezoelectric element out of noise included in a reception signal is expressed by Equation (1), and is increased as the electric capacitance C of the piezoelectric substance becomes smaller. Herein, k represents a Boltzmann constant, and T represents temperature (K).N=√{square root over (k·T/C)}  (1)
That is, a larger amount of noise of higher intensity is included in the reception signal output from the organic piezoelectric element compared to the reception signal output from the inorganic piezoelectric element, and thus a large amount of noise of high intensity is included in the ultrasound image produced based on the reception signals output from the organic piezoelectric elements. For this reason, although the ultrasound image produced based on the reception signals from the inorganic piezoelectric elements has an excellent S/N ratio, there is a problem in that, if the ultrasound image produced based on the reception signals from the organic piezoelectric elements is added thereto, the noise of high intensity included in the added ultrasound image is reflected, and the image quality of the resulting ultrasound image is unnecessarily deteriorated.